In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least an insulative photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by bringing an electrographic developer into contact with the latent image. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise discharge is brought about by the radiation-induced creation of pairs of negative-charge electrons and positive-charge holes, which are generated by a material (often referred to as a charge-generation material) in the electrophotographic element in response to exposure to the imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic potential and the types of materials included in the electrophotographic element, either the holes or the electrons that have been generated, migrate toward the charged surface of the element in the exposed areas and thereby cause the imagewise discharge of the initial potential. What remains is a non-uniform potential constituting the electrostatic latent image.
Among the many different kinds of materials known to be useful as charge-generation materials in electrophotographic elements are pigments such as titanyl fluorophthalocyanines. See, for example, U.S. Pat. Nos. 5,055,368 and 4,701,396 and copending U.S. patent application Ser. No. 07/533,634 (filed Jun. 5, 1990). Such pigments are known to be capable of generating electron/hole pairs in response to exposure to red and/or near-infrared radiation (i.e., radiation having significant intensity at a wavelength within the range of 600 to 900 nanometers). Such sensitivity to red and/or near-infrared radiation is especially useful when it is desired to sue light sources, such as light-emitting diode arrays or lasers, having major output in the red or near-infrared regions, to cause discharge of an electrically charged electrophotographic element.
It has also been recognized that generally known methods of synthesizing titanyl fluorophthalocyanines can yield crude forms of such pigments which are not as highly sensitive to red or near-infrared radiation as desired. The prior art has disclosed various methods of treating such crude pigments to improve their red and/or near-infrared sensitivity. For example. U.S. Pat. No. 4,701,396 disclose a method referred to therein as "acid-pasting", and U.S. Pat. No. 5,055,368 discloses a method that we will refer to as "salt-milling". Both of these methods are effective to improve the red or near-infrared photosensitivity of crude titanyl fluorophthalocyanines.
The disclosures of U.S. Pat. Nos. 4,701,396 and 5,055,368 also illustrate that when, after treatment by such referred-to methods, the pigments are dispersed in a coating solution of a polymeric binder and an organic solvent such as dichloromethane or trichloroethane, and the resulting coating composition is employed to form a photoconductive layer in an electrophotographic element, the electrophotographic element exhibits relatively high photosensitivity to near-infrared radiation.
Because of environmental concerns with the industrial use of certain solvents, such as chlorinated hydrocarbons (e.g., dichloromethane and trichloroethane), it would be desirable to form photoconductive layers from coating compositions containing other solvents, instead, such as, e.g., acetone, tetrahydrofuran, or alcohols such as methanol, ethanol, or 2-ethoxyethanol. However, the present inventors have found that if crude titanyl fluorophthalocyanines are treated to improve their red and near-infrared photosensitivity by methods such as the aforementioned acid-pasting or salt-milling processes and are then dispersed in a coating solution containing an organic solvent such as methanol or tetrahydrofuran, instead of a solvent such as dichloromethane or trichloroethane (sometimes alternatively referred to as "DCM" and "TCE", respectively), electrophotographic elements containing a photoconductive layer formed from such a coating composition will exhibit much lower photosensitivity to red and near-infrared radiation. The present inventors have found, further, that such adverse effects on photosensitivity are apparently caused by bringing the pigments into contact with a solvent such as methanol or tetrahydrofuran after they have been treated to improve their photosensitivity by a method such as, e.g., acid-pasting or salt-milling, because they have found that bringing the pigments into contact with tetrahydrofuran before acid-pasting or salt-milling does not adversely affect the photosensitivity achieved by subsequent acid-pasting or salt-milling. The present inventors have also found that the adverse effect of contact with a solvent such as tetrahydrofuran after acid-pasting or salt-milling appears to be relatively persistent. For example, the present inventors have found that if the pigment has been adversely affected by such contact, and the pigment is then removed from contact with the solvent that caused the adverse effect and is then dispersed in a coating solution of a polymeric binder and an organic solvent such as, e.g., dichloromethane or trichloroethane, which is then employed to form a photoconductive layer in an electrophotographic element, the electrophotographic element will still exhibit the adversely lower red and near-infrared photosensitivity.
Through further investigation, experimentation, and analysis, the present inventors have found that many other solvents, not just methanol or tetrahydrofuran (sometimes alternatively referred to as "THF"), will also cause the problem, e.g., other alcohols, acetone, N-methylpyrrolidone, diglyme, dioxane, N,N-dimethylformamide (sometimes alternatively referred to as "DMF"), pyridine, quinoline, morpholine, and ethylene glycol. More broadly, the present inventors have been able to characterize the "problem" solvents as organic solvents having a gamma.sub.c hydrogen bonding parameter value greater than 9.0. I.e., if a crude titanyl fluorophthalocyanine pigment as synthesized is subjected to acid-pasting or salt-milling to increase its red and near-infrared photosensitivity and is then brought into contact with an organic solvent having a gamma.sub.c hydrogen bonding parameter value greater than 9.0, its red and near-infrared photosensitivity will be significantly and persistently reduced.
The gamma.sub.c hydrogen bonding parameter value of an organic solvent is a measure of the proton-attracting power of the solvent. It is defined by J. D. Crowley, G. Teague, and J. W. Lowe in their paper entitled "A Three-Dimensional Approach to Solubility", published in the Journal of Paint Technology, Vol. 38, No. 496, May 1966, pp. 269-280, and has been accepted as a standard test of solvents, as described, for example, in the CRC Handbook of Solubility Parameters and Other Cohesion Parameters, by A. Barton, CRC Press, Boca Raton, Fla., 983, pp. 174 and 179-180 and in the ASTM D3132 standard test method.
Since many otherwise desirable coating solvents have a gamma.sub.c hydrogen bonding parameter value greater than 9.0, the present inventors were faced with the problem of providing electrophotographic elements containing a photoconductive layer formed from a coating composition, comprising a solution of such a solvent and a polymeric binder, having dispersed therein a titanyl fluorophthalocyanine pigment that has been acid-pasted or salt-milled to increase its photosensitivity, without substantially adversely lowering such increased photosensitivity.